The invention relates generally to sealed bearing earth boring drill bits, such as rotary cone rock bits, that utilize a fluid circulation medium. More particularly, the invention relates to such drill bits that include a protected lubricant reservoir.
More specifically, drill bits are generally known, and fall into at least two categories. Drill bits used for drilling petroleum wells and drill bits used in the mining industry are both well known in the art. While these two types of bits superficially resemble each other, the parameters that affect the operation of each are completely different. Petroleum drill bits typically use a viscous, heavy drilling fluid (mud) to flush the cuttings from the vicinity of the bit and carry them out of the hole, whereas mining bits typically use compressed air to achieve the same purpose. Petroleum bits typically drill deep holes, on the order of thousands of feet, and an average bit typically drills several hundreds or thousands of feet before being removed from the hole. In many instances, a petroleum bit is not withdrawn from the hole until it has exhausted its useful life. In contrast, mining bits are each used to drill several relatively shallow holes, typically only 30-50 feet deep, and must be withdrawn from each shallow hole before being shifted to the next hole. Thus, the effect of withdrawal and backreaming wear on the body of a mining bit are much more important considerations than they are for petroleum bits. In addition, because petroleum bits drill near the surface they are more frequently subjected to cave-ins, and must ream their way backwards out of the hole through the caved-in material. For these reasons, the factors that affect the design of mining bits are very different from those that affect the design of petroleum bits.
For instance, the viscosity and density of the drilling mud makes it possible to flush the cuttings from the hole even at relatively low fluid velocities. The air used to flush cuttings from mining holes, in contrast, is much less viscous and dense and therefore must maintain a rapid velocity in order to successfully remove the rock chips. This means that the cross-sectional area through which the air flows at each point along the annulus from the bit to the surface must be carefully maintained within a given range. Similarly, the rapid flow of air across and around a rock bit greatly increases the erosive effect of the cuttings, particularly on the leading portions of the bit.
Furthermore, rock bits are now being developed with sealed lubrication systems that allow easier rotation of the bit parts. These sealed lubrication systems typically comprise a lubricant reservoir in fluid communication with the bearings. In many cases, the reservoir is created by drilling a cavity into the bit leg. Access to the reservoir is through the installation opening of this cavity, which can then be sealed with a conventional plug or vented plug. These sealed lubrication systems are particularly vulnerable to erosion of the bit body, as any breach of the sealed system can result in the ingress of cuttings and/or particles into the bearings, causing bit failure. Heretofore, the reservoir opening has been located on the main outer face of each leg, with the result that the reservoir plugs and the walls of the reservoir itself are vulnerable to wear on the leg.
Hence it is desirable to provide a mining bit that provides increased protection for the reservoir and its installation opening and plug. It is further desired to provide a bit that is capable of withstanding wear on its shoulders and legs during backreaming or as the bit is being withdrawn from a hole.
In addition, it has been found that the pressure in the lubricant reservoir, and more particularly the pressure drop across the dynamic seals, can affect the performance of the dynamic seals and of the lubricant system in general. Hence, it has become desirable to control the fluid pressure in the lubricant reservoir. It is further desirable to do so without compromising the integrity of the sealed bearing system or rendering it vulnerable to excessive wear.